Volume 16, Number 1, Jan. 1994, pp17-20.
When thinking of Mexican art of the twentieth century the monumental mural cycles of Los Tres Grandes immediately spring to mind. The movement, which came into being with the 1921 commission for the National Preparatory School in Mexico City, produced some of the most compelling and dynamic images of this century in the Americas. Jose Clemente Orozco (1883-1949), David Alfaro Siqueiros (1898-1974), and Diego Rivera (1886-1957), the main protagonists, continually stressed the importance of mural painting within their oeuvre. In many cases this was for didactic purposes. Indeed, Siqueiros wrote,
"We repudiate all easel painting and every kind of art favoured by ultra-intellectual circles, because it is aristocratic, and we praise monumental art in all its forms because it is public property."1
However, throughout their careers these artists maintained a constant stream of easel paintings. Not only was this due to economic necessity but easel painting was also used as a means of working out ideas for compositions and as an experimental forum for techniques and materials to be later used on large scale mural projects. This was particularly true for both Siqueiros and Rivera; Orozco's methods and materials always tended to be more traditional.
Rivera and Siqueiros both had a conservative training at the Academy of San Carlos in Mexico City. They expanded their knowledge and experience of the latest trends in modern art through extensive travel and study abroad. Rivera was in Europe from 1907 to 1921. Most of his time was spent in Paris where he absorbed the ideas of the Cubist milieu.
The European influence on Los Tres Grandes is undisputed, but perhaps more important in thematic terms is the Precolumbian mural tradition. The early twentieth century was not just a period when Mexico was redefining itself politically, it was also a time when the country itself was opening up. A new, often politically motivated, interest in indigenous culture led to much archaeological exploration and the discovery of the fine mural cycles of Teotihuacan, Chichen Itza and Bonampak. Rivera and Siqueiros participated in several archaeological trips to strategic sights around Mexico City, the isthmus of Tehuantepec and the Yucatan in November 1921, at the very time that work was beginning on the Ministry of Public Education murals.
When Diego Rivera returned to Mexico from Europe in June 1921, almost immediately he began painting his first mural in the Bolivar Amphitheatre at the National Preparatory School. This mural, Creation, was executed in a wax technique with gold leaf additions. The success of the work inspired Leal, Revueltas and Siqueiros to experiment with the technique in the same building. However, although wax is relatively stable as a material, the technique used to apply it on such a large scale is slow and laborious consequently its use was soon dropped in favour of a more traditional fresco technique. Yet Rivera still continued using wax for easel paintings, experimenting with the medium throughout the 1920s. Two portraits of small Indian girls in the San Francisco Museum of Modern Art collection are representative of these experiments and will be discussed later.
According to Jorge Juan Crespo 2, one of the muralists, the paint was made up of beeswax, lemon resin (resine elemi) or copal, essence of lavender or turpentine and dry pigment. To prepare the paint the wax and lemon resin or copal were each mixed separately with the essence of lavender by boiling in a water bath or double boiler. The wax becomes a thick molasses like substance and is then ground thoroughly with the dry pigment with a smooth marble hand grinder. It could be thinned as desired with a few drops of lavender essence. The prepared colours were kept as a paste in air tight containers covered with water. The cement or stone wall was then prepared with the lemon resin mixture. If this was too thick and pasty to apply a blow torch was used to soften the material and make it more malleable. Blowtorches were also used to apply the paint to make it more fluid and to fuse the wax into the resin layer below.
Two different resins were used in this technique. The lemon resin is a European material and Rivera probably became familiar with it whilst working in Paris. In Mexico this material was found to be prohibitively expensive to obtain so Xavier Guerrero suggested substituting copal rosin.3 It is most likely that this is the resin component of the majority of the wax paintings done by the Mexicans and by Rivera in particular. Not only was copal more readily available and cheaper but it was an indigenous product still being used as a sacrificial incense by the Maya of the Yucatan. Indeed the very word copal is derived from the Nahuatl (Aztec) word copalli, which was their generic name for resins. It was used as a burnt offering in Precolumbian Mexico, a fact which no doubt appealed to Rivera's extreme nationalism.
Having established that experiments with wax continued throughout the 1920s in Rivera's easel painting oeuvre it is now important to consider the make up of this material and from this predict its long term ageing process and any conservation concerns this might reveal. The thinner for the paint was oil of spike lavender or turpentine. This was thought to evaporate leaving the oil or wax behind. However, essential oils like oil of spike lavender often leave a resinous residue within the paint which can oxidize and discolour. Indeed Doerner 4 mentions considerable darkening and blackening in his section dealing with the essential oils. Oil of spike lavender dries more slowly than turpentine, an advantage for its use in easel paintings where use of a blowtorch to keep paint more workable is impractical on a canvas support. The evaporation rate of these oils is also very slow due to being more resinous than turpentine, much slower even than the drying time of the fatty oils in oil paint. A combination of such oils, even in a wax vehicle, with oil paint, which is Rivera's technique at the time in question, could lead to serious cracking and flaking, particularly if the two layers are painted one over the other. Initially the oils would fuse but on drying the wax oil layers would reseparate and be susceptible to cracking and flaking.
The lemon resin component of the original recipe is an elemi. Elemi is a generic term for the large number of resins from the trees of the Burseraceae family.5 Manilla elemi is the one most frequently used in painting. Elemis are soft, plastic and malleable when they are fresh and are rich in essential oils, such as citrus oil, due to their high proportion of sesqui terpenes. Their function within such wax paint recipes is as a plasticizing agent to increase tackiness, particularly if bleached beeswax is used. Bleached beeswax is more dense and brittle than unbleached beeswax and so benefits from such a component, it is also the most common wax used for painting. However, such a high proportion of essential oils is a potential hazard for the paint layer as they can diffuse out of the paint layer leaving behind a layer that is harder and more prone to cracking and separation from under layers. They can also leave an opaque white crust on the surface. A soft copal will have a similar effect, it is often noted in artist's handbooks that a copal will create a paint film with a predilection for cracking.
The main component of the medium is beeswax. This is a fairly permanent material in its pure form but problems can occur within its structure on exposure to elevated relative humidity. In such cases the wax itself can disintegrate and partially perish by oxidation. Warmth is essential for the preparation of a wax colour; beeswax melts at 60-63°. C and therefore dissolves easily in turpentine, benzine or any fatty oil. This could be a potential problem and is one of the main objections to the use of wax with oil paint. This is because the ester link that forms wax is very similar to that of oil. There is therefore a high probability that wax could be a solvent for oil as it can so easily combine with other fatty oils given the right circumstances, for example the heat needed to apply it as a paint layer. In wax, palmitic acid, a saturated fat, a C16 chain, reacts with melissyl alcohol, a C30 chain, to form the ester melissyl palmitate. This is the major component of the wax, the ratio of melissyl palmitate to cerotic acid (a free fatty acid), the other major component of beeswax, is 6:1. Although oil is a more complicated structure than wax it also contains a significant proportion of saturated fats such as palmitic acid. Linseed oil can have up to 15% saturated fats. The linkage of glycerol and the three fatty acid chains to form the triglyceride which constitutes oil is also an ester linkage. It is therefore likely that wax with its high proportion of non-drying saturated fats can significantly change the properties of the oil paint with which it is in contact, leading to a much more susceptible paint film.6
With this knowledge of materials and technique let us now turn our attention to easel paintings that Diego Rivera was producing in the 1920s. The San Francisco Museum of Modern Art is fortunate enough to own two closely related works of small Indian girls painted in 1926 and 1927. In these two years Rivera was painting the monumental hymn to fertility in the chapel at Chapingo, the National Agricultural School. His pregnant wife, Guadalupe Marin, served as a model, his eldest daughter, Lupe, was two years old and his younger daughter, Ruth, was born in 1927. Thus it seems appropriate that with thoughts of germination and fruition on his mind he should turn his artistic attention to portraits of young children.
The two works in the SFMOMA collection are in extremely different states of repair. This could be due to Rivera modifying his technique or to the different histories that the two paintings have had. Although by this date Rivera had stopped using wax as a medium for fresco he was still experimenting with it on a smaller scale. There are many wax/oil paintings of this period and it seems that most exhibit similar problems.
All of the paintings that shall be considered are executed on heavy weight canvas, precluding the use of a blowtorch on the surface, therefore the fusion of the wax with an underlying layer does not happen. Instead wax is used in conjunction with tube oil colours to produce different textural and surface effects.
The earlier portrait of the "Indian Girl with Coral Necklace (Acc. No. 45.3004) has a much greater proportion of wax admixtures and the wax even seems to have been used in a significantly different manner to the later Young Girl Kneeling." The grey background area has the highest wax content of the image. It has been applied in a uniform manner over the red oil layer of the tiled floor beneath. This area has suffered considerable trauma and loss in the past, and extensive retouching is visible in ultraviolet radiation. This is due to both mechanical damage and the incompatibility of the oil/wax mixture with the different drying times and evaporation rates of the component parts. Indeed, this work exhibits much flaking in areas with wax additions, unlike the generally good surface of the later portrait where the technique is more straightforward. From a conservation point of view this has many implications, not least the fact that the use of wax makes the paint more sensitive to heat and solvent action and so precludes many normal treatments. There are areas of the work which seem to have suffered deformation from an applied heat source. The painting was purchased for $500 from Mrs. Sara Bard Field Wood on February 16, 1945. It had previously been on loan to the Museum since 1938. There were therefore only twelve years between execution and permanent exhibition by SFMOMA. Yet the first conservation record that exists for the work is from 1974 when the major damages had already occurred. It is therefore unknown when the damages and subsequent restoration took place and what their cause was. However, with the knowledge that the use of a wax resin layer over an oil layer can be problematic one could infer that this technique at least contributed to the fragile state of the painting and made it more susceptible to damage.
The later portrait of the Young Girl Kneeling (Acc. No. 91.179) has no ground layer with the oil and wax paint being applied directly on to the sized canvas. This has resulted in an opaque, matte tonality overall where the oil paint has been absorbed into the interstices of the fabric support. Although most of the painting has been done alla prima there is some reworking of the image, most notably in the yellow background which has been painted over to cover part of the skirt and the diamond lattice pattern. Aesthetically the use of wax is more controlled in this image. It is used with great effect to highlight by creating a glossier surface. This is particularly effective in the head and hair of the girl where the wax additions recreate the shimmering quality of light on shining hair. Where wax is used it is applied as an upper layer over an oil paint base. There seems to have been little mixing of the paint unlike in the earlier image. Examination under stereo microscopy reveals crisp delineated brushstrokes that appear to have had little effect on the paint beneath. It can be inferred that Rivera had learnt to leave a longer drying time between the two layers to avoid the problems encountered in earlier works.
A whitish crystalline bloom is also found in several areas of the earlier SFMOMA piece, most notably in the blue background to the left of the girl, which also seems to have some waxy addition to the paint. This is seen in the different surface sheen in this area and also its more marked sensitivity to wax dissolving solvents such as benzine. Due to the fragility of the paint surface this layer has proven impossible to remove safely. The layer has formed because the essential oil component of the wax has migrated out of the paint film.
Can any form of interventive conservation help to remedy these problems? In the past treatments have been undertaken on some of these works to try and stabilize them and protect the surface from future damage. The SFMOMA Indian Girl with Coral Necklace underwent major treatment in 1974. At this time the surface was cleaned with a 1% tri-sodium phosphate solution, losses were filled and retouched with pigments ground in Acryloid B67 and finally a protective coating of Acryloid B67 was sprayed over the entire surface to protect against airborne grime. At this period Rivera never varnished his work. Although the B67 varnish was applied with the best of intentions, to preserve the existing state of the surface, in practical terms it has created more problems for the surface. Discolouration of the paint and retouching has continued and the retouching is now apparent to the naked eye. This cannot be removed as the solvents needed to remove the B67 affect the original paint. Hence the overall coat of B67 varnish cannot be removed either. This varnish significantly alters the play of light on the work and masks the original differences in surface sheen seen so beautifully in Young Girl Kneeling. It compromises the surface and therefore the fundamental aesthetics of the piece.
If Teas solubility charts are compared of beeswax and Acryloid B67 there seems to be a range of solvents that might conceivably remove the retouching and varnish without compromising the original paint. These solvents are heptane, acetone, butan2one and propan2ol. However, careful and controlled testing proved their use ineffective, they either had no effect or had the capability to dissolve everything. This is obviously because in the painted matrix we are not dealing with pure substances in isolation. The materials are complicated by the addition of essential oils and resins which again change the solubility thresholds of the composite piece.
In contrast the Young Girl Kneeling has a pristine surface. It had been in the Elise S. Haas collection since it was painted and came to SFMOMA as a bequest in 1991. There has been no varnishing and no interference in the structure of the work at all, in it we see a work almost as it would have left Rivera's studio. Although the painting had a heavy layer of surface grime it was possible to remove this without affecting any of the unvarnished paint. A 1% solution of tri-ammonium citrate was used as the cleaning agent followed by a deionized water wash. The image brightened dramatically, although it is still a fairly low key painting due to the technique used. There is no ground layer so the paint has sunk, the canvas is coarse and so the surface scatters the light and lowers the tonal values of the piece. Although the surface is vulnerable to damage due to the softness of the wax additions other preventative conservation methods will be considered for protection rather than irreversibly altering the surface by applying a coat of varnish.
It is important to minimize contact with the surfaces of all these works and to keep them in low temperatures for both storage and exhibition. Preventative measures are extremely important as treatment options are severely limited for these pieces. The paintings should have protective backings to prevent trauma from the rear which could prove unrestorable from the front. Glazing would also be useful particularly if the works were to travel. Backing and glazing would ensure physical protection and also create a stable microclimate to guard against swings in relative humidity and temperature. There is little to be done in terms of undoing past intervention therefore protection of those works that are still relatively free of restoration is crucial if we are to have an understanding of how the artist himself viewed them on completion.
1. A History of Mexican Mural Painting, by Antonio Rodriguez, pg 199.
2. Modern Mexican Painters, by Laurence E. Schmeckebier, pg 40.
3. Art Making from Mexico to China, by Jean Charlot. Xavier Guerrero, Aztec Artist, pg 143.
4. Materials of the Artist, by Max Doerner, pg 121.
5. Painting Materials, by Rutherford J. Gettens and George Stout, pg 21.
6. Painting Materials, by Rutherford J. Gettens and George Stout, pg 78.
7. Materials for Conservation, by C.V. Horie, pgs 204, 218.
Timestamp: Thursday, 11-Dec-2008 13:02:33 PST
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